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Abstract:

A printed dynamic optical illusion printed on a printing device using a
plurality of colorants, wherein one or more of the colorants are
appearance mutable colorants having spectral characteristics that can be
controllably switched between a first colorant state and a second
colorant state by application of an appropriate external stimulus, and
wherein one or more mutable portions of the optical illusion image are
printed using at least one appearance mutable colorant. The mutable
portions are controllable such that when they are in a first appearance
state the printed optical illusion image has a first illusion state, and
when they are in a second appearance state the printed optical illusion
image has a second illusion state, thereby changing the optical illusion
image from the first illusion state to the second illusion state so as to
affect the perception of an optical illusion by a human observer.

Claims:

1. A printed dynamic optical illusion image printed by a printing device
on a print media using a plurality of colorants, wherein one or more of
the colorants are appearance mutable colorants having spectral
characteristics that can be switched between a first colorant state and a
second colorant state by application of an appropriate external stimulus,
and wherein one or more mutable portions of the printed dynamic optical
illusion image are printed using at least one appearance mutable
colorant; wherein when the mutable portions are in a first appearance
state the printed dynamic optical illusion image has a first illusion
state, and when the mutable portions are in a second appearance state the
printed dynamic optical illusion image has a second illusion state, such
that changing the optical illusion image from the first illusion state to
the second illusion state affects the perception of an optical illusion
by a human observer; the printed dynamic optical illusion image being
switchable between the first and second illusion states by applying the
appropriate external stimulus to switch the one or more appearance
mutable colorants between their first and second colorant states, thereby
switching the mutable portions of the printed dynamic optical illusion
image between their corresponding first and second appearance states.

2. The printed dynamic optical illusion image of claim 1 wherein the
changing of the dynamic optical illusion image from the first illusion
state to the second illusion state causes the optical illusion to become
visible, alters the visual impact of the optical illusion or introduces
one or more additional optical illusions.

3. The printed dynamic optical illusion image of claim 1 wherein the
dynamic optical illusion image includes one or more immutable portions,
and wherein the first or second appearance state for at least one of the
mutable portions provide a color match to one of the immutable portions.

5. The printed dynamic optical illusion image of claim 1 wherein the
external stimulus is provided by a controlled stimulus source or an
ambient stimulus source.

6. The printed dynamic optical illusion image of claim 5 wherein the
controlled stimulus source provides by controlled dosage of the applied
stimulus, with the dosage of the controlled stimulus is controlled by
controlling an intensity, an exposure time, a retention time, a duty
cycle, a direction or a modulation of the external stimulus.

7. The printed dynamic optical illusion image of claim 6 where the dosage
is controlled to produce an intermediate colorant state intermediate to
the first and second colorant states, thereby providing an intermediate
appearance state for the mutable portions intermediate to the first and
second appearance states.

8. The printed dynamic optical illusion image of claim 1 wherein the one
or more appearance mutable colorants include a photochromic colorant
having spectral characteristics that can be switched by an external
stimulus that is an optical radiation stimulus.

12. The printed optical dynamic illusion image of claim 8 where the
optical radiation stimulus is provided by an ambient light source,
including room lights or daylight.

13. The printed dynamic optical illusion image of claim 1 wherein the one
or more appearance mutable colorants include a thermochromic colorant
having spectral characteristics that can be switched by an external
stimulus that is a thermal stimulus.

14. The printed dynamic optical illusion image of claim 13 wherein the
thermal stimulus is a change in an ambient temperature, and wherein the
printed dynamic optical illusion image is used to provide an
environmental sensing function.

16. The printed dynamic optical illusion image of claim 1 wherein the one
or more appearance mutable colorants include an electrochromic colorant
having spectral characteristics that can be switched by an electrical
stimulus.

17. The printed dynamic optical illusion image of claim 1 wherein at
least one of the appearance mutable colorants is reversible such that it
can be repeatedly switched between its first and second colorant states.

18. The printed dynamic optical illusion image of claim 1 wherein at
least one of the appearance mutable colorants is irreversible such that
it can only be switched from its first colorant state to its second
colorant state a single time.

19. The printed dynamic optical illusion image of claim 1 wherein one or
more of the colorants are immutable colorants having spectral
characteristics are constant, and wherein portions of the optical
illusion image that are not mutable portions are printed using the
immutable colorants.

20. The printed dynamic optical illusion image of claim 19 wherein at
least one of the mutable portions of the printed optical illusion image
is printed using a combination of colorants including at least one
appearance mutable colorant and at least one immutable colorant.

21. The printed dynamic optical illusion image of claim 1 wherein the
printed optical illusion image includes one or more human recognizable
textual or symbolic messages that can be perceived by a human observer
when the one or more mutable portions are in at least one of their first
or second appearance states.

22. The printed dynamic optical illusion image of claim 21 wherein at
least one of the human recognizable textual or symbolic messages follows
a contour of image content within the printed dynamic optical illusion
image.

23. The printed dynamic optical illusion image of claim 1 wherein the
printed optical illusion image includes one or more machine readable
textual or symbolic messages that can be perceived by a machine vision
device when the one or more mutable portions are in at least one of their
first or second appearance states.

24. The printed dynamic optical illusion image of claim 23 wherein at
least one of the machine readable textual or symbolic messages follows a
contour of image content within the printed dynamic optical illusion
image.

25. The printed dynamic optical illusion image of claim 1 wherein the
first and second illusion states differ with respect to a field of view,
spatial frequency content, patterns, or contours of the optical illusion.

26. The printed dynamic optical illusion image of claim 1 wherein one or
more of the mutable portions are outside an area occupied by the optical
illusion.

27. The printed dynamic optical illusion image of claim 1 wherein the
first colorant state and the second colorant state of the appearance
mutable colorants differ in a shape or amplitude of the spectral
characteristics.

28. The printed dynamic optical illusion image of claim 1 wherein the
first appearance state and the second appearance state of the mutable
portions differ in at least one of a hue, saturation or lightness color
appearance characteristic.

29. The printed dynamic optical illusion image of claim 1 wherein the
first appearance state and the second appearance state of the mutable
portions differ in image contrast or image content.

30. The printed dynamic optical illusion image of claim 1 wherein the
first colorant state or the second colorant state for at least one of the
mutable colorants is a clear state, colorless state, nearly colorless
state or a fully colored color state.

32. The printed dynamic optical illusion image of claim 1 wherein the
printing device is an inkjet printer, an electrophotographic printer, an
offset litho printer, a dry offset printer, a letterpress printer, a
gravure printer, a flexography printer or a screen printer.

33. The printed dynamic optical illusion image of claim 1 wherein the
printed optical illusion image is included in an advertisement or an
advertising display or is used in product packaging.

34. The printed dynamic optical illusion image of claim 1 wherein the
printed optical illusion image is used for consumer testing, for
cognitive or visual testing, for steganography, for a reverse Turing
test, or as a CAPTCHA.

36. The printed dynamic optical illusion image of claim 35 wherein the
physiological optical illusion is a color perception optical illusion or
an apparent motion optical illusion that produces the appearance of
motion in a static image.

[0002] This invention pertains to the field of optical illusion images and
more particularly to the creation and use of dynamic optical illusion
images, whose appearance changes when subjected to an external stimulus.

BACKGROUND OF THE INVENTION

[0003] In considering the human visual system, most people immediately
think about the eyes and the constituent parts, such as the cornea,
pupil, eye lens, the vitreous humor, and the retina with the rods and
cones. However, the human visual system also includes neural processing
that enables image interpretation and understanding. Such neural
processing generally occurs automatically, without any conscious
consideration by the observer. For example, the images formed on the
retina of each eye are upside down, but the visual system automatically
provides orientational corrections. As another example, the ratios of the
number of short or blue cones, the number of medium or green cones, and
the number of long or red cones varies widely among individuals, as does
the organization of these cones within the retina, whether randomly
dispersed or clustered. The green cones are also much more light
sensitive than the red or blue cones. Yet, by and large, most people
perceive the various color shades in a sufficiently comparable way that
we can agree upon the colors present in a scene.

[0004] The structure and response attributes of the human visual system
impact perception; for example, in cinema, the temporal processing limits
allow people to perceive continuous motion from a series of still frame
images presented at 30 frames/sec (fps). By comparison, human perception
of optical illusions exploit gaps or expectations in our automatic
cognitive visual system, such that we can perceive visual content or
sensations within the image content which is often not actually present
in the content itself. An optical illusion is characterized by the visual
perception of image content that differ from objective reality. The
information gathered by the eye is processed in the brain to give a
perception that does not completely correlate with a physical measurement
of the stimulus source or image. Certainly, the perception of optical
illusions varies on an individual basis, depending on visual sensitivity
and impairments (such as color blindness). The perception of optical
illusion can also depend on age and cultural influences. For example,
children tend not to perceive the well-known Ebbinghaus illusion as
strongly as adults do, while non-Western and rural people, who are less
accustomed to interpreting depth and flatness cues from two-dimensional
(2D) images, can be less susceptible to the well-known Ponzo illusion.
While human observers often respond to optical illusions attentively and
with puzzlement or bemusement, some illusions can induce discomfort or
nausea in certain observers.

[0005] Optical illusions can be characterized to include cognitive optical
illusions, in which the eye and brain make unconscious inferences, and
physiological illusions, in which the eyes and brain are affected by
excessive stimulation of a specific type (e.g., brightness, tilt, color
or motion). Examples of cognitive illusions include illusions of
perspective, such as the Penrose Stairs or the drawings by M. C. Escher.
These images exploit our expectations of how a three-dimensional view is
illustrated in two-dimensions, by cleverly misapplying cues of
perspective and shading to depict objects that are physically impossible.
The Ponzo illusion similarly exploits our expectations that convergent
lines are associated with distance; to have two lines intersecting the
convergent lines appear of different length, even though their lengths
are identical.

[0006] While many physiological optical illusions have been created, their
perceptive impact on the human visual system has not been fully
explained. The article "Uncertainty in visual processes predicts
geometrical optical illusions", by Cornelia Fermuller et al. (Vision
Research, Vol. 44, pp. 727-749, 2004) proposes that uncertainty or noise
in the human visual system that relates to determining intensity,
positions, and orientations of image features introduces systemic errors
that cause perceptual anomalies compared to the original image. For
example, interpretive biases relative to perception of edge elements and
intersection points may explain many geometrical illusions, while biases
in interpreting motion or optical flow may cause motion illusions such as
the Ouchi illusion (see FIG. 6A). Generalizing, it is noted that there
are many physiological optical illusions, including the well-known
Hermann Grid and scintillating grid illusions, or the well-known pulsing
vortex, rotating snakes, and flowing leaves illusions, that can be
classified as apparent motion optical illusions which provide temporally
variant sensations, such as pulsation and movement, including rotation,
expansion, and contraction, that are not actually present in the images
themselves, which are static in nature. Such illusions can be observed
whether they are printed or presented statically on an electronic
display. The temporal effects of these illusions can generally be
attributed to unconscious interpretive bias in image interpretation that
intersects with temporal phenomena of the human visual system such as
peripheral drift, neural lags, fixation and after image effects, and
saccadic eye motions.

[0007] There are also physiological optical illusions that specifically
exploit attributes of human color perception, such as color and contrast
adjacency effects, to fool the eye in seeing colors and grayscale patches
incorrectly. The article, "A Brief Classification of Colour Illusions`,
by A. Kitaoka (Colour: Design & Creativity, Vol. 5, pp. 1-9, 2010), gives
many examples of color perception optical illusions. For example, there
are illusions of color or contrast differences that exploit adjacency
interactions that are embedded in human lateral signal processing. These
illusions reveal perceptual difference in color or contrast perception of
an area when adjacent areas have different luminosities or color content.
For example, with the Munker-White illusion, the perception of mid-tone
neutral (grey) patches is altered by the presence of strong luminance
shifts (darker or lighter) in neighboring image content. The Munker
illusion illustrates a similar effect on color perception, where the
perception of identical color patches is altered by the presence of
different adjacent colors near one color patch as compared to another.

[0008] While some optical illusions, such as the Ouchi illusion or the
Fraser-Wilcox illusions, are motion illusions that have a perceptual
impact as black and white or grayscale images, their effects can be
enhanced when color is also used. For example, A. Kitaoka has published
integrated illusions, such as peripheral drift illusions and the
"rotating snakes" illusion, which combines color selection with other
illusions, such as the optimized Fraser-Wilcox and the Rotating Ouchi
illusion to create illusions that can be more visually compelling than
the black and white parent illusions. The paper "The effect of color on
the optimized Fraser-Wilcox illusion", by A. Kitaoka (9th L'OREAL Art and
Science of Color Prize, pp. 1-16, 2006) discusses how color and contrast
manipulation can change the visual impact of the optimized Fraser-Wilcox
illusion, including changing the direction and rate or magnitude of the
apparent rotation of the image. The paper "Illusory motion from change
over time in the response to contrast and luminance", by B. Backus et al.
(Journal of Vision, Vol. 5, pp. 1055-1069, 2005) proposes that optical
illusions of motion are generated when viewing static repeated asymmetric
patterns due to fast and slow changes over time in the neuronal
representation of contrast or luminance. The impact of other temporal
illusions, such as the pulsing vortex illusion, or the Hermann grid and
scintillating grid illusions, the latter of which cause the perception of
pulsing at the intersections of a white (or light-colored) grid on a
black background, can also be altered by changing the pattern of colors
and contrast within the illusionary image.

[0009] At present, optical illusions are generally available as
collections in books or on websites, and their design and physiological
basis is discussed in academic papers. In general, optical illusions are
primarily used to amuse or entertain viewers, including in art and
architecture. With their unusual visual trickery, optical illusions often
hold an observer's attention for prolonged time periods, while the
observer is both amused and puzzled by the visual effects. Optical
illusions are also used as tools in vision research to test or reveal the
cognitive or perceptual mechanisms, interactions, or clinical
deficiencies of the human visual system.

[0010] Otherwise, the human perception of optical illusions has been used
for little practical effect, although their selective use as a "reverse
Turing test", has been proposed in the paper "Practical Application of
Visual Illusions: errare humanum est" by G. Brelstaff et al (Proc. 2nd
Symposium on Applied Perception in Graphics and Visualization, p. 161,
2005). A Turing test is a test of a machine's ability to exhibit
intelligent behavior, and a machine passes the test if its function, at
least in a limited way, is indistinguishable from that of a human. By
comparison, a reverse Turing test is a test to distinguish humans from
computers and other forms of artificial or alien intelligence. The one
widely used version of a reverse Turing test practiced today, involves
the use of a "CAPTCHA", a "Completely Automated Public Turing test to
tell Computers and Humans Apart", as a visual word recognition
challenge-response test. CAPTCHAs are widely used in computing to attempt
to ensure that a response is generated by a human rather than a computer.
For example, as humans can readily interpret the confusing image content
of a CAPTCHA, while computer algorithms have difficulty, humans can gain
website access while automated programs are restrained. The Belstraff
paper proposes that as humans innately can perceive optical illusions,
such as Kitaoka's Rotating Snakes, and machine vision systems cannot,
that the human perceptual reaction to static optical illusion images can
be used to discriminate the presence of human intelligence. As a variant,
it has also been proposed to use text illusions as CAPTCHAs, as human
readable steganography, in which smaller case static text is hidden
within larger case static text.

[0011] Thus, the opportunity exists to present, create, and alter optical
illusion images for various practical and previously unforeseen effects.
In particular, printed optical illusion images which are dynamic by the
use of mutable inks can provide novel functional value.

SUMMARY OF THE INVENTION

[0012] The present invention represents a printed dynamic optical illusion
image printed by a printing device on a print media using a plurality of
colorants, wherein one or more of the colorants are appearance mutable
colorants having spectral characteristics that can be switched between a
first colorant state and a second colorant state by application of an
appropriate external stimulus, and wherein one or more mutable portions
of the printed dynamic optical illusion image are printed using at least
one appearance mutable colorant;

[0013] wherein when the mutable portions are in a first appearance state
the printed dynamic optical illusion image has a first illusion state,
and when the mutable portions are in a second appearance state the
printed dynamic optical illusion image has a second illusion state, such
that changing the optical illusion image from the first illusion state to
the second illusion state affects the perception of an optical illusion
by a human observer;

[0014] the printed dynamic optical illusion image being controllably
switchable between the first and second illusion states by applying the
appropriate external stimulus to controllably switch the one or more
appearance mutable colorants between their first and second colorant
states, thereby switching the mutable portions of the printed dynamic
optical illusion image between their corresponding first and second
appearance states.

[0015] This invention has the advantage that the printed optical illusion
image can be controlled to dynamically adjust the visual impact of the
optical illusion.

[0016] It has the additional advantage that changing the visual impact of
the optical illusion can be used to attract the attention of an observer.

[0028] FIGS. 6A and 6B depict two illusion states of a printed dynamic
optical illusion image based on the Ouchi illusion; and

[0029]FIG. 7 is a flowchart illustrating a method for forming a printed
optical illusion image in accordance with the present invention.

[0030] It is to be understood that the attached drawings are for purposes
of illustrating the concepts of the invention and may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The invention is inclusive of combinations of the embodiments
described herein. References to "a particular embodiment" and the like
refer to features that are present in at least one embodiment of the
invention. Separate references to "an embodiment" or "particular
embodiments" or the like do not necessarily refer to the same embodiment
or embodiments; however, such embodiments are not mutually exclusive,
unless so indicated or as are readily apparent to one of skill in the
art. The use of singular or plural in referring to the "method" or
"methods" and the like is not limiting. It should be noted that, unless
otherwise explicitly noted or required by context, the word "or" is used
in this disclosure in a non-exclusive sense.

[0032] The present invention involves a form of intelligent printing that
provides dynamically-changing printed optical illusion images. Optical
illusion images have the characteristic that they are eye catching.
Dynamically-changing optical illusion images are even more eye catching.
This makes them valuable for applications such as advertising displays
and product packaging where the goal is to draw the attention of a
potential customer. They can also be useful for many other applications
including watermarking, steganography, security, and the study of human
perception and cognition.

[0033] Within the content of the present invention, a dynamically-changing
optical illusion is a printed optical illusion image whose content is
dynamically changed as a function of time, either in whole or in part, in
response to an appropriate external stimulus. In particular, within an
image or document including an optical illusion image, the visual design
of the optical illusion image, or part thereof, including color and
contrast, is changed to make the perceptual experience for an observer of
the optical illusion image more or less effective, or altogether
different, or to hide or reveal other image or informational content
hidden in or near the optical illusion image.

[0034] In one embodiment, the external stimulus that is used to
dynamically modify the printed optical illusion image is an optical
radiation stimulus that provides illuminating radiation. For example, a
dynamically-changing optical illusion image can be included on the
packaging for a particular product. The product can then be displayed in
a store and a nearby optical radiation source can be modulated, to change
the appearance of the optical illusion image with time, thereby
attracting the attention of a potential customer to the image and the
accompanying product. In general, the visual response of observers to a
dynamically-changing optical illusion image can be used to either attract
their attention to the images themselves or to accompanying messages, or
to distract the observers from other considerations, including
accompanying messages.

[0036] In the original version of the static Walter Anthony optical
illusion image 400 of FIG. 1A, image regions 420 which are reproduced as
mid-tone grey were deep purple in color, and image regions 422 which are
reproduced as dark grey were deep green in color. Whereas image regions
424 that are shown as white, and image regions 426 that are shown as
black are reproduced essentially as originally depicted. The optical
illusion image 400, as reproduced in FIG. 1A, with grays, black and
whites, generally provides a very small, if any, optical illusionary
effect for most observers. Some observers may perceive a slight pulsing
drift motion in the center of the image. By comparison, if the image
regions 420 are reproduced with dark purple, and the image regions 422
are reproduced with dark green as with the original Walter Anthony
version, many observers experience a very strong optical illusion effect,
where the central portion of the pattern seems to drift inwards while the
outer portion of the pattern seems to drift outwards, particularly when
it is viewed with peripheral vision. For some observers, the perceived
motion effect can be repetitive, with jumpy transitions, where perception
of the image motion seems to reset.

[0037] In one exemplary embodiment of the present invention, the optical
illusion image 400 is dynamically altered to become a dynamic optical
illusion image 410 wherein the image regions 420 and 422 are mutable
portions that can take on a plurality of appearance states. The image
regions 420 and 422 are printed with appearance mutable colorants having
spectral characteristics that can be controllably switched between first
colorant states and second colorant states by application of an
appropriate external stimulus. For example, when the mutable colorants in
the mutable portions are in their first colorant states, the image
regions 420 and 422 can take on a highly chromatic appearance (e.g, dark
purple and dark green, respectively), corresponding to first appearance
states, such that the optical illusion image 400 takes on a first
illusion state providing a strong optical illusion effect. Alternately,
when the mutable colorants in the image regions 420 and 422 are in their
second colorant states, the mutable image regions 420 and 422 take on
second appearance states, which can be neutral, less chromatic, or
chromatically shifted in appearance, thereby producing a second illusion
state of the dynamic optical illusion image 400. In this second illusion
state, the optical illusion image 400 can have a weak, or even
negligible, optical illusion effect.

[0038] In this hierarchy of image mutability, a change in the colorant
states of the mutable colorants in one or more mutable portions of a
dynamic optical illusion image 400 changes the appearance of the one or
more mutable portions (e.g., image regions 420 and 422) from a first
appearance state to a second appearance state. This provides a
corresponding change in the overall appearance of the dynamic optical
illusion image. The different states of the dynamic optical illusion
image can be referred to as illusion states. Changing the mutable
portions from their first appearance state to their second appearance
state provides a corresponding change in the dynamic optical illusion
image from the first illusion state to the second illusion state. In many
cases, the change in illusion states corresponds to a change in the
perceptual impact of the optical illusion. In other instances, the change
in the illusion states can correspond to the appearance or disappearance
of hidden text, messages, images, or codes within or adjacent to the
optical illusion portion of the dynamic optical illusion image (with or
without changing the state or perceptual impact of the optical illusion).
As another example, a change in the illusion states can also leave a
primary optical illusion image intact, but add a secondary illusion
effect to the dynamic optical illusion image 400. Changes in the
perceptual impact of the optical illusion can also occur in combination
with changes in the appearance of hidden content. The changes in illusion
states can be either subtle or dramatic. In some cases, the entire
dynamic optical illusion image may be a mutable portion such that the
entire image area can be changed when the colorant states of the mutable
colorants are changed.

[0039] In the above example, the spectral absorption characteristics (or
likewise, the spectral reflectance characteristics or the spectral
transmittance characteristics) of the mutable colorants is controlled
between a low-saturation state and a high saturation state. By adjusting
the shape or amplitude of the spectral absorption characteristics, it is
possible to adjust various color appearance characteristics of the
mutable portions of the optical illusion image, such as the hue,
saturation or lightness. In some embodiments, the spectral fluorescence
characteristics of the mutable colorants can be controlled such that the
mutable colorants have more or less fluorescence, or fluoresce with
different spectral absorption/emission characteristics. In other
embodiments, the mutable portions of the optical illusion image can be
controlled to adjust image attributes such as image contrast or image
content.

[0040] As another variant, rather than controlling the image regions 420
and 422 between first colorant states having a highly chromatic
appearance and second colorant states having a neutral appearance, one or
both of the image regions 420 and 422 can be controlled between two
chromatic colorant states. For example, the image regions 420 can be
controlled between a first colorant state and a second green colorant
state which matches the green color present in image regions 422. When
the image regions 420 are in the second green colorant state, the optical
illusion image 400 exhibits a greatly reduced perceptual optical illusion
effect relative to the case when the image regions 420 are in the first
purple colorant state, similar to the visual effect produces when the
image regions 420 and 422 are controlled to have a neutral appearance.
For example, it has been found that an intermediate level of the optical
illusion effect can be observed if the colorant state for the image
regions 420 is controlled to provide a brown colorant state, whereas a
strong level of the optical illusion effect can be observed if the
colorant state for the image regions 420 is controlled to provide a red
colorant state. Therefore, it can be seen that by proper selection of an
appropriate pair of colorant states for one or more of the image regions
in the optical illusion image 400, it is possible to control the
perceptual impact of the optical illusion effect to produce the desired
results. Additionally, although the changes to the mutable portions can
change image content of a optical illusion portion of a dynamic optical
illusion image, such as by changing its perceptual impact, such changes
can provide other effects, including providing alternate or secondary
optical illusions (such as causing a second optical illusion portion to
appear) or altering various appearance attributes of the optical illusion
(such as adding an illusionary scintillation effect to an existing
optical illusion image) or hiding or revealing hidden text or patterns.

[0041] To further illustrate the effect of selectively altering optical
illusion images, a cognitive dynamic optical illusion image 410F in the
style of M. C. Escher is shown in FIGS. 2A-2C, having a two-story gazebo
structure 510 and a walkway 505. In particular, FIG. 2B illustrates a
classical M. C. Escher style optical illusion 400 of an impossible
building in which the walkway 505 appears to be traverse in a contiguous
fashion from the lower tower level to the upper tower level of the gazebo
structure 510, even though ramps or stairs are lacking. In FIG. 2A, a
dynamic optical illusion image 410F includes a mutable portion 520 that
can be controllably switched between a first appearance state where it
matches the color of the walkway 505 and a second appearance state where
it matches the color of the gazebo structure 510. In this way, the
cognitive optical illusion switches between illusion states by
application of an external stimulus.

[0042] In that context, FIG. 2B illustrates the appearance of the dynamic
optical illusion image 410F when the mutable portion 520 has been
controlled to match the color of the walkway thereby providing a first
illusion state 500A. In this first illusion state 500A, the dynamic
optical illusion image 410F depicts an impossible structure where the
continuous walkway 505 presents an impossible structure.

[0043]FIG. 2c illustrates the appearance of the dynamic optical illusion
image 410F when the mutable portion 520 has been controlled such that the
resulting appearance state matches the color of the gazebo structure 510
thereby providing a second illusion state 500B. In this second illusion
state 500B, the apparent paradox is broken so that the walkway 505 no
longer appears to connect to the second story of the gazebo structure
510. The walkway 505 appears to always be on ground level and pass behind
the gazebo structure 510.

[0044] Accordingly, it is seen that careful adjustment of an image can
introduce (or remove) an optical illusion, or can alter the perceptual
impact of an optical illusion for an observer. Specifically, portions of
an optical illusion image can be adjusted to affect any of its
attributes, including but not limited to content, color (hue, saturation
or lightness) and contrast to enhance or reduce the illusionary effect.
To that end, FIG. 3A depicts an exemplary system in which a data
processor 150 is connected to an electronic display 120 having a screen
130 and a printer 100 for printing on print media 110 using a plurality
of colorants, including one or more mutable colorants having spectral
characteristics that can be controllably switched between a first
colorant state and a second colorant state. The plurality of colorants
will also generally include one or more conventional immutable colorants
whose spectral characteristics are static. The data processor 150 can
provide image data to the display 120 or the printer 100, including image
data for optical illusion images 400 or dynamic optical illusion images
410. The dynamic optical illusion images 410, having one or more mutable
portions that can be altered to dynamically change the appearance or
presence of the optical illusion, can be provided to the printer 100 as
printed dynamic optical illusion images 410G or to the display 120 as
dynamic optical illusion images 410H. The dynamic optical illusion images
410G and 410H can also include adjacent image content 407 that may or may
not be mutable. This system can include other components such as
processor-accessible memory, communications controllers, input/output
devices, and other hardware not shown.

[0045] When the dynamic optical illusion image 410H is presented on the
display 120, the mutable portions can be controlled by dynamically
switching back and forth between two different versions of the optical
illusion image where the mutable portions have been adjusted
appropriately. Alternately, the mutable portions of the image can be
adjusted in real time by altering the content of the memory used to store
the image data for the dynamic optical illusion image 410H. The display
120 can be used for a user to preview the appearance of the dynamic
optical illusion image 410H during the process of designing and
optimizing the optical illusion effect, and for example, compare it to
the original static optical illusion image 400, to determine if the
desired transformation and perceptual effects are being achieved.
Preferably, the previewed dynamic optical illusion image 410H is designed
to simulate the appearance of a printed dynamic optical illusion image
410G that will be produced by the printer 100, accounting for the
spectral characteristics of the mutable and immutable colorants available
in the printer.

[0046] In the case of printed content, most colorants such as inks,
toners, pigments, dyes, or other materials that can be printed onto the
print media 110 to provide informational or image content are not dynamic
once they have been deposited and cured or dried. Normal colorants have
static spectra characteristics, aside from being subject to degradation,
such as fade due to UV exposure, or other abuse (such as smearing). Thus,
it is particularly difficult to dynamically change printed content.

[0047] There are however certain special classes of mutable colorants that
can be printed, then controllably stimulated to induce changes in their
spectral characteristics. Mutable colorants are chromogenic materials,
which are materials that change their color upon application of an
appropriate stimulus. Types of mutable colorants include thermochromic
colorants, photochromic colorants, and electrochromic colorants. Such
colorants can be controllably switched between a first colorant state and
a second colorant state by application of an appropriate external
stimulus. Thermochromic colorants have spectral characteristics that can
be controllably switch by a thermal stimulus. Photochromic colorants have
spectral characteristics that can be controllably switch by an optical
radiation stimulus. Likewise, electrochromic colorants can reversibly
change between coloration states and transparency (changed spectral
characteristics) when subjected to an electrical stimulus such as an
applied voltage. However, thermochromic and photochromic colorants have
the advantage that they enable a dynamic response without requiring the
presence of imbedded or printed electronics. There are a wide variety of
thermochromic and photochromic colorants that are commercially available,
including both reversible colorants that can be cycled back and forth
between the different colorant states, and irreversible colorants that
can only undergo a single state change (e.g., from clear to colored).

[0048] It is noted that other forms of mutable colorants, such as
fluorescent colorants, photoluminous (glow-in-the dark) colorants,
piezochromic (pressure sensitive) colorants, hydrochromic (moisture
sensitive) colorants, or halochromic (pH sensitive) colorants, can also
be used for various embodiments. In some applications, the mutable
colorants can be mixed or over-coated with other colorants or protective
materials.

[0049] Thermochromic inks that can be used for mutable colorants in
various embodiments of the present invention are commercially available
from many companies, including Chromatic Technologies International of
Colorado Springs, Colo., LCR Hallcrest of Glenview, Ill., and Printcolor
Screen Ltd. of Berikon, Switzerland. Typically, thermochromic inks have a
first colorant state producing a first visible color at a nominal
temperature level. Then when the thermochromic inks are heated (or
cooled) by application of an appropriate thermal stimulus their spectral
characteristics change to a second colorant state producing a second
visible color. In some cases, one of the colorant states may be a clear
(i.e., invisible) colorant state where only a minimal amount of the
incident light is absorbed by the colorant. Some common types of
thermochromic dyes are based on mixtures of leuco dyes with suitable
other chemicals that display a color change (usually between the
colorless leuco form and the colored form) in dependence on temperature.

[0050] As an example, LCR Hallcrest offers a range of mixed temperature
thermochromic inks that respond under different temperature conditions.
These include cold activated thermochromic inks which undergo a color
change when cooled (progress from clear to colored at or below 15°
C.), thermochromic inks that change color at body temperatures (progress
from colored to clear at 31° C.), and thermochromic inks that
change color in response to high temperatures (progress from colored to
clear at 47° C.).

[0051] Photochromic inks are also available from many companies, including
Chromatic Technologies International and LCR Hallcrest. Typically
photochromic inks are invisible (clear or colorless) or lightly colored
(nearly colorless or pale) until illuminated with light (typically UV or
short wavelength blue), and then become fully colored colors which are
much more saturated colors, including pastel or fully saturated colors,
once stimulated with the appropriate light dosage (intensity, wavelength
or spectrum, and time).

[0052] Photochromic and thermochromic color changes are distinct from
fluorescence induced color changes, as the color changes do not require a
continuous stimulus, but are metastable and can linger over extended
periods of time (e.g., minutes or hours). Response times vary, but the
changes can be relatively quick (within seconds or less). Some
photochromic dyes have been experimentally used as optical switches with
switching times of ˜1 μs. For example, some LCR Hallcrest
photochromic inks become fully colored colors that are intensely colored
after only 15 seconds exposure to direct sunshine and then return to
clear after about 5 minutes indoors. Photochromic inks can fade to clear
at various rates. For example, the LCR Hallcrest orange and yellow inks
are relatively slow to return back from their colored state to their
clear state. The prolonged or metastable color states can be particularly
advantageous for the present invention, because typically humans perceive
optical illusion images more slowly than they do normal image content.
Additionally, for many applications, such as smart packaging, the
temporal pattern of an occasional stimulus and a prolonged response can
be advantageous. The ink lifetime can be limited because of the
degradation from solar UV exposure, but UV protected inks are becoming
available.

[0053] While thermochromic and photochromic inks often produce pastel or
unsaturated colors, they still span a range of hues, as for example,
thermochromic inks can change from colorless, or near colorless, states
to fully colored colors that are red, orange, green, blue, purple,
magenta, or other colors, depending on the formulation. Photochromic inks
can experience similar color changes. The color gamut which can be
achieved can be expanded by printing these mutable inks in combination
with normal non-mutable inks Published papers, such as "Chromatic
Properties of Thermochromic Inks", by L. Johansson (TAGA Proceedings,
2006) and "Thermochromic Inks--Dynamic Colour Possibilities", by R.
Kulcar et al., (Proc. CREATE Conference, pp. 202-206, 2010), provide some
detail about the color changes obtained with these types of inks during
screen printing experiments.

[0054] Examples of electrochromic colorants that could be used in
accordance with the present invention would include NanoChromics inks
available from NTERA of Philadelphia, Pa. These inks combine
nanocrystalline and color change materials for applications such as smart
packaging and flexible displays.

[0055] FIG. 4A, which is taken from the Johansson paper, shows a graph 300
illustrating a range of color changes, as characterized in the well-known
CIELAB color space, that are achieved by thermochromic inks from New
Prismatic Enterprise Co. Ltd. of Taiwan for yellow, green, red, blue,
violet, and other color inks. The arrows indicate the direction of color
shift as the ink samples are heated and progress between endpoint colors,
from the fully colored color of a first colorant state 350 to nearly
colorless color at a second colorant state 355. In FIG. 4A, an example
using a Yellow ink or colorant is shown, which also has an intermediate
colorant state 360 in addition to the endpoint colors.

[0056]FIG. 4B, which is taken from the Kulcar paper, shows a graph 302
illustrating trajectories the color of reversible red, blue and green
thermochromic inks as a function of temperature, using inks from Coates
Screen Inks GmBh (Germany) and Sicpa (Switzerland). As can be seen from
both FIGS. 4A and 4B, reversible thermochromic inks typically lose color
during heating (progressing from first colorant state 350 through
intermediate color states 360 to second colorant state 355) and regain it
during cooling. At the first colorant state 350, the cold temperature
colors or fully colored colors can be pastel in color, or fully saturated
colors (or nearly so), depending on the colorant. Both Figures also
indicate that decolorization may not be complete, and that typically a
pale or yellowish tint remains in the heated state or second colorant
state 355. The gap between the cooling and heating color trajectories in
FIG. 4B is indicative of color hysteresis.

[0057]FIG. 4c, also reproduced from Kulcar, shows a scale expanded graph
304 for the blue ink that illustrates the hysteresis experienced by
reversible thermochromic inks in greater detail. This hysteresis can be
substantial, as color trajectories traced out from a first colorant state
350 (cool) to a second colorant state 355 (heated) through the CIELAB
color space pass through different intermediate color states 360 during
heating than during cooling. Thus, the visibility of color and image
content can be manipulated to provide intermediate color states, not just
the endpoint color states (such as hot and cold temperatures). Thus
intermediate appearance states, involving illusionary image content or
hidden patterns or messages, and intermediate illusion states, can occur.
However, the hysteresis can make the use of intermediate colorant states
360 more difficult or predictable. For this reason, the use of controlled
stimulus, whether to consistently drive the mutable colorants to endpoint
color states, or reliably to intermediate colorant states 360) or color
matches between various mutable image portions or between mutable and
immutable image portions, can be valuable. The control of stimulus dosage
to attain only endpoint color states is likely less demanding than the
control required to reliably provide intermediate color states, within
appropriate colorimetric tolerances. It is noted that the responsiveness
of thermochromic inks can also be shown in other ways, including as
graphs in lightness (L*) vs. temperature. For brevity, comparable color
space diagrams and response curves for photochromic inks are not
provided.

[0058] The color changes shown in FIGS. 4A-4C occur because of changes in
spectral absorption/reflectance/transmittance characteristics of the inks
in response to the applied thermal stimulus. For, example, the paper
"Colorimetric properties of reversible thermochromic printing inks", by
Kulcar et al. (Dyes and Pigments, Vol. 86, pp. 271-277, 2010), provides
spectral data for a red thermochromic ink from Coates Screen GmBh. FIG.
4D shows a graph 306 depicting changes in the ink spectral reflectance
for this thermochromic ink as a function of temperature. At an initial
temperature (a), the unactivated spectral reflectance profile 310 is high
in the red, and low in the green and blue; this corresponds to a red
color appearance for a first colorant state 350. When the ink is heated
to state (b), the activated spectral reflectance profile 312 is high
across the whole visible range, although it is slightly lower in the
blue. In this state the ink has a clear appearance (with a slight yellow
tint) for a second colorant state 355. When the ink is cooled to state
(c), which is still warmer than state (a), but is somewhat cooler than
state (b), the green reflectance has dropped moderately (corresponding to
an increase in the green spectral absorbance). In this state, the ink has
an intermediate colorant state 360 that is light yellow to light orange
in appearance. After further cooling to state (d), the reflectance in the
blue and green has dropped significantly, and the intermediate colorant
state 360 has a more pronounced reddish coloration.

[0059] As another example, FIG. 4E shows a graph 308 depicting the changes
in light absorption for the Reversacol Flame photochromic ink from
Vivimed Labs of Hyberbad, India. It can be seen that an unactivated
spectral absorption profile 320 is quite flat and low (having a near
neutral tint), which equates to an unactivated resting state, or first
colorant state 350, of the ink. With application of an ultraviolet
optical radiation stimulus at a wavelength of 380 nm, the high UV
absorption of the ink will produce a strong activation of the dye. Once
activated, maximum absorption occurs at around 475 nm in the blue region
of the spectrum, producing a new activated spectral absorption profile
322. Removal of the blue fraction of the spectrum by the dye results in
the ink having a second colorant state 355 that has a red-orange color.

[0060] Thus, it can be seen that chromogenic materials or mutable
colorants, including thermochromic and photochromic inks, can produce a
wide range of controllable color changes when used individually. The
range of color change can be expanded when these inks are used in
combination, including combining multiple types of thermochromic or
photochromic inks (either reversible or irreversible), or combining
photochromic inks with thermochromic inks, or combining mutable inks or
with normal non-mutable inks. These abilities can be used for the present
purpose to create and controllably alter mutable portions of dynamic
optical illusion images 410 between multiple appearance states, thereby
providing one or more illusion states, such that the perceptual impact of
an optical illusion image 400 can be changed for an observer. Depending
on the properties of the respective inks, combination printing can be
achieved by over-coating or patterning, such as with halftone or
continuous tone dots. Other properties, such as dot gain differences or
transparency, can determine which inks are preferentially printed first,
and which are printed later, and are nearer the top surface of the
printed surface. It should be understood that the term "ink", for the
purposes of the present invention, is a pigmented liquid or paste used
especially for writing or printing, which includes a colorant, a solvent,
a vehicle and additives, as appropriate. Mutable inks are often
micro-encapsulated with a color former (dye), a color developer within
the microcapsules, and binders between them. Other types of mutable
colorants can also be used in accordance with the present invention, such
as mutable toners for use in electrophotographic printing systems.

[0061] Returning to a discussion of FIG. 3A, the system having the data
processor 150 is connected to the printer 100 to produce a printed page
including the printed dynamic optical illusion image 410G on the print
media 110. The printer uses a plurality of colorants, including one or
more appearance mutable colorants having spectral characteristics that
can be controllably switched between a plurality of different colorant
states by application of an appropriate external stimulus. The data
processor 150 receives image data specifying the image content for the
dynamic optical illusion image 410H. This mutable optical illusion image
can be designed, using knowledge of image science and color science to
provide the image content including one or more mutable portions that
will have the intended appearance for the different illusion states
associated with the different colorant states of the one or more mutable
colorants. The data processor 150 determines printer image data
specifying the amounts of the various colorants as a function of position
that is then provided to the printer 100 to produce the printed dynamic
optical illusion image 410G. The printer image data is designed to
provide the desired image content producing the intended illusion
effects, while achieving appropriate image quality attributes, including
color matches, color gamut, color accuracy, the presence of intermediate
colors, metamerism, resolution, or contrast. The image quality attributes
can be described by various parameters, including color space coordinates
and just noticeable differences (JNDs). The printed dynamic optical
illusion images 410G can be controlled using the appropriate external
stimulus to provide the desired illusion states when the mutable inks are
at endpoint color states (e.g., first colorant state 350 and second
colorant state 355 in FIG. 4A), intermediate color states (e.g.,
intermediate colorant state 360 in FIG. 4A), or combinations thereof.

[0062] The printer 100 shown in FIG. 3A can be a desktop or mid-market
printer, such as an inkjet printer that uses one or more inkjet print
heads (not shown) to print on the print media 110, which can include cut
sheet paper. As another example, FIG. 3B depicts a high speed printer 160
that deposits ink on a fast moving web print media 165. Although the web
print media 165 is generally meant to be paper, it should be understood
that the method of the present invention for creating and using printed
dynamic optical illusion images can print the images on any appropriate
type of media or substrate, which include, but are not limited to, paper,
cardboard, cloth or other textiles, plastic or polymer surfaces or
substrates (transparent or opaque), glass, metal sheet, multi-layer
composite materials, or variations and combinations thereof.

[0063] As depicted, the exemplary printer 160 has 4 print stations 170,
although it may have more. In this example, the first three print
stations 170c, 170m, and 170y, apply cyan, magenta, and yellow inks,
respectively, and the fourth print station 170M applies a mutable ink.
Alternately, these print stations can deposit different colored inks,
such as black, red, green, or blue inks. In some embodiments, one or more
additional print stations can be added to apply additional inks such as a
black ink or another mutable ink. Mutable ink is applied by one or more
print stations 170M, which can apply any type of mutable ink, including
the thermochromic or photochromic inks that were discussed earlier. FIG.
3C then shows a simplified ink jet print head 175 that can be provided
within a print station 170 for the case where the printer 160 is a
continuous inkjet printer. The print head 175 includes multiple nozzles
177 arranged in an array. For continuous inkjet printers, ink flows
continuously through the print head and provides streams of fluid, which
break into ink droplets 180. The droplets are either allowed to fall onto
the web print media 165 (FIG. 3B), or are deflected to an ink catcher for
locations where no ink should be applied to the printed page.

[0064] In other embodiments, the print head 175 can be a drop-on-demand
print head which only produces drops as they are needed. In some
configurations of the printer 160 in FIG. 3B, some of the print stations
170 can use continuous-type inkjet print heads 175, while other print
stations can use drop-on-demand print heads 175. For example, the print
stations 170c, 170m and 170y can use continuous-type inkjet print heads,
while the print station 170M can use a plurality of drop-on-demand print
heads, such as, for example, one or more for each type of mutable ink to
be applied. As mutable inks tend to have higher viscosity than many
normal (non-mutable) process inks, the print heads 175 for print station
170M are more likely to have ink ejector mechanisms, such as ones having
piezo actuators, to provide the greater force necessary to emit such
inks. However, it should be understood that multiple kinds of inks,
including both normal and mutable (reversible or irreversible) inks, can
also be emitted by the same nozzle, or printed with the same applicator,
either simultaneously as mixtures or sequentially. It should also be
understood that while FIG. 3c specifically illustrates the use of ink jet
printing technology; other technologies can also be used to apply the
mutable inks for the purposes of the present invention. For example,
printer 160 can use a variety of printing technologies including
electrophotographic, offset litho, dry offset, letterpress, gravure,
flexography, or screen printing.

[0065] Returning to a discussion of FIG. 1A, which depicts a previously
discussed optical illusion image 400. As was mentioned earlier, in one
exemplary embodiment, the optical illusion image 400 is a printed optical
illusion image, and particularly a dynamic optical illusion image 410
with image regions 420 and 422 that are mutable portions printed with
appearance mutable colorants. According to one embodiment, the black
image regions 426 are printed with an immutable black colorant and no
colorants are printed in the white image regions 424. For example, the
mutable image regions 420 are printed with a light gray color using the
immutable black colorant, and are overprinted with a first mutable
colorant that can be controllably switched between a clear colorant state
and a purple colorant state (e.g., the Violet mutable colorant shown in
FIG. 4A). Similarly, the mutable image regions 422 are printed with a mid
gray color using the immutable black colorant, and are overprinted with a
second mutable colorant that can be controllably switched between a clear
colorant state and a green colorant state (e.g., the Green mutable
colorant shown in FIG. 4A). The mutable colorants are changed between the
first and second states by application of an appropriate external
stimulus. The external stimulus, for example, can be a thermal stimulus
for the case where the mutable colorants are thermochromic colorants, or
an optical radiation stimulus for the case where the mutable colorants
are photochromic colorants. When the mutable colorants are in their clear
colorant states, the image regions 420 and 422 will have a neutral
appearance, and the optical illusion image 400 will take on a first
illusion state having a very weak visual impact for a typical observer.
But when the mutable colorants are switched to their purple and green
colorant states, the optical illusion image 400 will take on a second
illusion state having a strong visual impact for a typical observer. As
another example, image regions 420 and 422 can be printed with immutable
colorants having a light or mid tone blue color, and over printed with
mutable colorants such as the Yellow and Red colorants of FIG. 4A. When
the mutable inks are in their colored states, over the underlying blue
colorants, then purple and green colorant states can appear in image
regions 420 and 422 respectively.

[0066] As another embodiment of the present invention, a dynamic optical
illusion image 410A is shown in FIG. 1B. In this example, the printed
optical illusion image includes a central mutable portion 430 that is
printed with mutable colorants, and an outer immutable portion 435 that
is printed with conventional immutable colorants. In this case, the shape
of the central mutable portion 430 follows a natural contour 460 in the
optical illusion pattern, although this is not a requirement. When the
mutable portion 430 is in a first appearance state, the mutable colorants
can be either colorless (as shown in FIG. 1B) or colored, providing a
first illusion state where the optical illusion has a very weak visual
impact for a typical observer. When the mutable inks are switched to take
on second colorant states, the mutable portion 430 takes on a second
appearance state, which can correspond to a second illusion state where
the optical illusion has a strong visual impact for a typical observer.
For example, the central mutable portion 430 can be printed with one or
more photochromic inks that are colorless, or nearly so, when
unactivated. Then when an activating optical radiation dosage is applied,
these inks transition to colored states, completing the optical illusion
image. As a result, in one illusion state, the optical illusion within
the dynamic optical illusion image 410A will have a strong visual impact
for a typical observer. In particular, the anomalous motion that was
described earlier can be perceived in the dynamic optical illusion image
410A. In the other illusion state, the visual impact of the optical
illusion is modified or eliminated, depending on the color and contrast
content changes, particularly in the central mutable portion 430.

[0067] In another embodiment, the central mutable portion 430 of the
dynamic optical illusion image 410A in FIG. 1B can be printed with a
combination of mutable and immutable colorants. For example, as was
discussed earlier with reference to FIG. 1A, the mutable portion can be
printed using a neutral pattern printed with an immutable black ink or
blue ink overlaid with mutable inks that can be switched between a clear
state and a colored state. In this way, the color content of the mutable
portion 430 can be more or less visible depending on the state of the
accompanying mutable inks.

[0068] FIG. 1C depicts another exemplary dynamic optical illusion image
410B, which is similar to the dynamic optical illusion image 410A of FIG.
1B, except that the positions of the mutable portion 430 and the
immutable portion 435 have been reversed. In this case the central
immutable portion 435 is printed with conventional immutable colorants,
and the outer mutable portions 430 or corners are printed using mutable
colorants, or combinations of mutable and immutable colorants. In this
case, the transition between mutable and immutable image regions follows
part of the image pattern or contours 460. In a first illusion state, the
mutable colorants can be clear (as shown in FIG. 1B) or can take on some
other first state colors where the optical illusion has a first visual
impact. In a second illusion state, the mutable inks are controllably
switched to take on second state colors where the optical illusion has a
different visual impact. Dynamically modifying the mutable colorants in
this way can significantly affect the perception of the anomalous motion
present in the original Walter Anthony optical illusion image because the
peripheral image content is changing. For simplicity, the examples of
FIGS. 1B and 1C show portions of the original illusion image being
removed. However, it should be understood that changes in the illusion
image content can be more subtle, and that the illusion pattern,
contours, or spatial frequencies of patterns can also be changed.

[0069] FIG. 1D depicts another exemplary dynamic optical illusion image
410C, where a left immutable portion 435 is printed with conventional
immutable colorants, and a right mutable portion 430 is printed using
mutable colorants, or combinations of mutable and immutable colorants. In
this case, the boundary between the mutable portion 430 and the immutable
portion 435 does not follow natural contours in the optical illusion
pattern. In one embodiment, the mutable portion 430 is printed with
reversible thermochromic color change inks that vanish when activated to
temperatures above body temperature by an applied heat stimulus, thereby
interfering with the visual impact of the optical illusion. When the
dynamic optical illusion image 410C is subsequently cooled, the visual
impact of the optical illusion is restored. Dynamic optical illusion
images 410C that respond to temperature changes can provide a visually
compelling approach to environmental sensing. For example, they can be
used to provide a novel and interesting way to provide a visual
indication of whether the temperature of a product, such as a cold drink,
falls within a preferred temperature range.

[0070] FIG. 1E depicts another exemplary dynamic optical illusion image
410D, which includes a central mutable portion 430 and an outer immutable
portion 435, similar to FIG. 1B. In this example, one or more hidden
patterns 450 (i.e., the text characters "M" "W" and "A") can be revealed
in at least one state of the mutable ink. The hidden patterns 450 can be
printed with immutable ink(s) or mutable ink(s), or combinations thereof.
For example, photochromic inks that are colorless, or nearly so, when
inactivated can be used to print the optical illusion pattern in the
mutable portion 430, while immutable inks can be used to print the hidden
pattern 450. When the mutable inks are in their colorless state, the
hidden pattern 450 is revealed. Then when stimulating light is applied,
the mutable inks transition from their colorless states to colored states
to provide the complete optical illusion pattern and obscure the
visibility of the hidden pattern 450. In some embodiments, the hidden
text can be printed with a color pattern that matches the revealed
optical illusion pattern so that the hidden pattern 450 blends in with
the optical illusion pattern and is essentially invisible. In some
embodiments, the hidden pattern 450 is printed with mutable colorants
that can be controlled to change to a colorless state at the same time
that the mutable inks used to print the optical illusion pattern are
controlled to change to their colored state. The hidden pattern 450 can
represent an advertising message, a watermark, a security code, a bar
code, or any other type of message.

[0071] FIG. 1E also illustrates that the hidden pattern 450 can be readily
apparent, and appear in the middle of the image as shown by the "M"
character. Alternately, the content of the hidden pattern 450 can be more
subtly positioned and designed to follow the patterning or contours 460
of the optical illusion image as shown by the "W" and "A" characters. In
the latter case, the hidden pattern 450 can follow the contours 460 at an
edge or transition between mutable portion 430 and the immutable portion
435 of the dynamic optical illusion image 410D.

[0072] In other embodiments, the hidden pattern 450 can be a mutable
pattern embedded in the dynamic optical illusion image 410D. For example,
most of the dynamic optical illusion image 410D can be static (i.e.,
printed with normal immutable inks) or partially static (i.e., printed
with mixtures of immutable inks and mutable inks), such that a
perceptually impactful optical illusion image is present at all times. In
this case, the hidden pattern 450 is printed predominantly with mutable
inks that can experience dramatic color changes in response to an
appropriate external stimulus. As a result, the mutable portion 430 of
the dynamic optical illusion image 410D of FIG. 1E can change between
first and second appearance states, corresponding to first and second
illusion states that change the appearance or perceptual impact of the
optical illusion, or change between appearance states that change the
appearance of the hidden pattern 450, or combinations thereof. In such
instances, the hidden pattern 450 can become visible to an observer in at
least one of the colorant states, but detection of the hidden pattern can
still be difficult due to the text or code being fully embedded in the
optical illusion pattern (including following contours 460), and due to
the ongoing presence of part or all of an optical illusion pattern
distracting the observer. As such, the dynamic optical illusion image
410D can be an example of steganography, which is the art and science of
writing hidden messages (text or code) in such a way that no one, apart
from the sender and intended recipient, suspects the existence of the
message; a form of security through obscurity. As a variant, the dynamic
optical illusion image 410D can be designed to help direct the observer
towards the hidden pattern 450 when it appears.

[0073] FIG. 1F depicts another exemplary dynamic optical illusion image
410E that responds to a patterned stimulus 440 that is applied in a
spatially variant manner. For example, the patterned stimulus 440 can be
accomplished with structured lighting or heating. As one example, the
entire dynamic optical illusion image 410E can be printed with at least
mutable colorants, such that the full extent (length and width or area)
of the image can be changed with an appropriate stimulus. However, a
localized change to the dynamic optical illusion image 410E can be
accomplished if a patterned stimulus 440 is selectively applied according
to a structured irradiation pattern. In this case, only the stimulated
portions of the dynamic optical illusion image 410E will be changed. In
the example shown in FIG. 1F, the patterned stimulus 440 is applied in a
striped pattern that does not follow the contours of the optical illusion
pattern. In other embodiments, the patterned stimulus 440 can be applied
using a pattern that is related to the optical illusion pattern, and for
example, involves contour matching.

[0074] In some embodiments, the dynamic optical illusion image 410E can
include some image regions that use mutable colorants (or combinations of
mutable and immutable colorants) and are subject to modification using a
patterned stimulus 440, and other image areas that use only immutable
colorants and are therefore unaffected by the patterned stimulus 440. In
this example, the stimulus can be applied in a spatially variant manner
to selectively change the image content in image regions that use the
mutable colorants, while leaving the image areas that use only immutable
colorants unaffected. For example, the image regions that use the mutable
colorants can be printed with photochromic inks that transition from
colorless (or pale) color states to colored states when a patterned light
stimulus is applied to activate the inks.

[0075] Various combinations of structured and unstructured stimuli are
also possible. For example, a first stimulus (e.g., light of a first
spectral band) can be uniformly applied to the whole image to cause
changes in large image areas, while a second stimulus (e.g., light of a
second spectral band) can be applied selectively to cause changes in
particular image areas.

[0076] Returning to a discussion of FIGS. 2A-2C, which depicts the Escher
style illusion, the dynamic optical illusion image 410F can be printed
using various arrangements of mutable and immutable colorants. In one
embodiment, the walkway 505 and the gazebo structure 510 are printed with
immutable colorants, and the mutable portion 520 is printed with a
combination of immutable and mutable inks. For example, the gazebo
structure 510 can be printed with a yellow immutable ink, and the walkway
can be printed with a combination of yellow and cyan immutable inks so
that it has a green color. The mutable portion 520 can then be printed
with a combination of a yellow immutable ink that matches the color of
the gazebo structure 510, and a photochromic mutable ink that can be
controllably switched between a clear state and a cyan state with the
application of an appropriate external optical radiation stimulus.

[0077] When the photochromic mutable ink is in its unactivated clear
state, the mutable portion 520 will have a yellow color matching the
color of the gazebo structure 510. In this case, the dynamic optical
illusion image 410F has the appearance shown in FIG. 2c, where there is
no paradox in the appearance of the walkway 505. When an appropriate
stimulating light is applied so that the photochromic mutable ink is
switched to its activated cyan state, the mutable portion 520 will have a
green color matching the color of the walkway 505. In this case, the
dynamic optical illusion image 410F has the appearance shown in FIG. 2B,
where the walkway paradoxically appears to be contiguous from the lower
tower level to the upper tower level even though it lacks a ramp or
stairs. This represents an example of a cognitive optical illusion image.

[0078]FIG. 5A depicts an observer 210 in a viewing environment 215
interacting with a printed item 200 that includes a printed dynamic
optical illusion image 410G. A stimulating radiation source 222 provides
an optical radiation stimulus 220 to cause a state change in the mutable
inks that are used to print the mutable portions of the printed dynamic
optical illusion image 410G, thereby providing two or more illusion
states. The optical radiation stimulus 220 can include visible light, or
invisible radiation such as ultraviolet or infrared radiation. As shown,
the optical radiation stimulus 220 is provided by radiation source 222
that is driven by a controller (not shown) within a system that provides
controlled illumination of the printed dynamic optical illusion image
410G. Alternately, the optical radiation stimulus 220 can be sunlight or
ambient light, which further can be modulated by a mechanical blade. For
at least some illusion states, an optical illusion pattern is revealed in
the printed dynamic optical illusion image 410G that then can
perceptually impact the observer 210. Visible light 227 in the viewing
environment 215 can originate from a light source 225, or can be ambient
light including daylight, or combinations thereof. The light source 225
can be a tungsten light source, a fluorescent light source, light
emitting diodes (LEDs) or any other kind of light source including a
black light.

[0079] In some embodiments, the printed dynamic optical illusion image
410G is viewed by the observer as part of a visual experiment, or a
marketing study. In this case it is useful for an image capture device
230 such as a digital camera to be included in the viewing environment.
This enables the response of the observer 210 to be monitored as the
illusion state of the printed dynamic optical illusion image 410G is
varied. This monitoring can be conducted to determine emotional or
physiological responses to the optical illusion images or accompanying
messages (including advertising), or to test or reveal responses related
to vision or cognitive research.

[0080] In a preferred embodiment, the stimulating radiation source 222
provides an optical radiation stimulus 220 that has little or no
visibility to the observer 210. In various embodiments, the optical
radiation stimulus 220 can be UV radiation (for example having a
bandwidth of 300-380 nm or 360-380 nm), low wavelength blue light (for
example having a wavelength of ˜420 nm), or infrared radiation (for
example having a bandwidth of 800-960 nm) as is appropriate for safely
activating or deactivating the printed dynamic optical illusion image
410G. In other embodiments, the stimulating radiation source 222 can
provide high intensity visible light 227, or narrow bandwidth laser
light, as the optical radiation stimulus 220.

[0081] In some embodiments, the printed dynamic optical illusion image
410G can be made using mutable UV responsive inks that are essentially
sensitive only to short wavelength UV radiation having wavelengths<300
nm. In this case the optical radiation stimulus 220 should include
optical radiation at the corresponding wavelengths. This has the useful
advantage that the mutable inks will not be activated by the UV radiation
in atmospheric-filtered solar radiation.

[0082] Although most photochromic colorants or materials are stimulated to
exposure by UV light, versions sensitive to IR light have been reported
in the literature. In one embodiment of the present invention,
photochromic inks which have at least one stimulative bandwidth that
resides between ˜1400-1500 nm are used and activated by the
appropriate IR optical radiation stimulus 220. These inks have the
advantage that they cannot be accidently stimulated by daylight, because
the spectral profile of atmospherically filtered solar radiation lacks
significant light in that spectral band. Alternately, other IR spectral
bands, spanning ˜1150-1200 nm, at ˜980 nm, and at ˜790
nm can be used, although the atmospheric filtering is progressively less
effective as the stimulating wavelength drops towards the visible
wavelength range. Of course, there are also applications, including
artwork or environmental sensing, where it can be advantageous to use
photochromic inks that respond to ambient light, including visible light,
daylight, or general room lights.

[0083] As was discussed with reference to FIG. 1F, it is also noted that
the stimulating radiation source 222 can use the techniques of structured
illumination to direct a patterned optical radiation stimulus 220 onto
the printed dynamic optical illusion image 410G. The pattern of optical
radiation can be stationary or can be scanned across the printed dynamic
optical illusion image 410G. In general, the intensity of a controlled
optical radiation stimulus 220 can be specified in units of watts (W) or
lumens of optical flux, or in terms of irradiance (e.g., mW/cm2), or
energy (J), or energy density (e.g., mJ/cm2) delivered, with
appropriate tolerances (e.g., ±5%) including illumination uniformity
or pattern specifications, as appropriate.

[0084] The mutable portions of the printed dynamic optical illusion image
410G can also be activated by other types of external stimulus or by
combinations of different types of stimuli, which can include a pressure
stimulus (e.g., provided by a touch or by sound waves), or a thermal
stimulus from a heat source. If thermal stimulation is to be applied
broadly, the stimulating radiation source 222 can be used as a physically
distant heat source that provides a heat stimulus 228. In some
embodiments, a thermal stimulus can be applied using other type of heat
sources (e.g., using a resistive heater array) positioned closer to the
printed item 200. For example, the printed item 200 can be positioned on
top of a surface that contains the controlled heat source. This approach
enables the provision of a patterned thermal stimulus. For environmental
sensing type applications, the printed dynamic illusion image 410 can
respond to a thermal stimulus according to ambient heat levels. The
thermal stimulus can be specified in terms of the flux, flux density,
energy, or energy density delivered.

[0085] In some embodiments, the printed dynamic optical illusion image
410G can be used to attract the attention of a consumer 212 in a retail
viewing environment 217 as shown in FIG. 5B. In this example, a system
for presenting the printed dynamic optical illusion image 410G to an
observer 210, having sources of external stimulus and a controller
thereof, is shown. For example, this system can be a retail product
display, including a product display arrangement for displaying retail
products packaged in product packaging 240, and an advertisement display
250. Both the product packaging 240 and the advertisement display 250
include printed dynamic optical illusion images 410G. It will be
recognized that in some embodiments only one of the product packaging 240
or the advertisement display 250 will include the printed dynamic optical
illusion images 410G. The product display arrangement shown in FIG. 5B
includes a shelf for stacking the retail products. However, it will be
recognized that the retail products can be displayed with any type of
product display arrangement known in the art including, but not limited
to, display racks, hooks, bins and refrigerators or freezers.

[0086] A controller 260 is used to provide a control signal to control the
stimulating radiation source 222 in order to provide the appropriate
stimulus as required to change the appearance states of the mutable
portions, thereby changing the illusion states of the printed dynamic
optical illusion images 410G. In particular, controller 260 controls the
dosage of the external stimulus applied to the dynamic optical illusion
images 410G in order to cause a change in the colorant states of the
mutable colorants. The controlled parameters can include light dosage
(e.g., intensity, exposure time or modulation, spectral bandwidth,
direction or patterning) or heat dosage (e.g., intensity, exposure time,
direction, or modulation), where the dosages and dosage tolerances can be
appropriate to provide endpoint colors or intermediate colors. In some
embodiments, the controller 260 controls the stimulating radiation source
222 to automatically switch the printed dynamic optical illusion images
410G between first and second illusion states according to a predefined
timing pattern. For example, the illusion states can be switched every 30
seconds. In some embodiments, the controller 260 controls the stimulating
radiation source 222 to switch the printed dynamic optical illusion
images 410G between the different states in response to user activation
of a user interface control such as a button, a switch, a pointing device
(e.g., a mouse or a trackball), or a touch sensitive display.

[0087] The inclusion of printed dynamic optical illusion images 410G in
the product packaging 240 can take different forms in various
embodiments. For example, the printed dynamic optical illusion images
410G can be printed on a box or other container used to enclose the
retail product, it can be printed onto a surface of the retail product
itself, it can be included on a printed label that is attached to or
inserted into the product packaging 240, or it can be included in any
type of printed product packaging known in the art.

[0088] The inclusion of the printed dynamic optical illusion images 410G
in the advertisement display 250 can also take different forms in various
embodiments. For example, printed dynamic optical illusion images 410G
can be included in posters (e.g., movie posters), retail end-cap
displays, store shelf displays, signage, an advertising brochure, an
advertisement in a printed publication (e.g., a magazine or a newspaper),
or any other type of advertisement display 250 known in the art.

[0089] The retail display also includes a stimulating radiation source 222
for providing the optical radiation stimulus 220 used for controllably
switching the illusion state of the printed dynamic optical illusion
images 410G. The stimulating radiation source 222 can be controlled to
switch between illusion states on a regular interval in order to attract
the attention of the consumer 212 in order to increase the likelihood
that a product is purchased. In other embodiments, different forms of
external stimuli can also be incorporated into the retail viewing
environment, such as thermal heat sources.

[0090] The portrayal of dynamic optical illusion images also has other
applications. For example, as the illusion state of a dynamic optical
illusion image 410 is changed, a hidden pattern 450 can appear, including
being imbedded or contour following, as shown in FIG. 1E. Color change
effects, including contrast or lightness changes or saturation, can hide
or reveal other illusionary content, or other images, or hidden patterns
450, from image layers accompanying the dynamic optical illusion image
410A. The observer can examine a printed dynamic optical illusion image
410G (FIG. 3A) or a dynamic optical illusion image 410H (FIG. 3A) on a
display 120, during which time the optical illusion(s) can either attract
or distract the attention of an observer from the hidden pattern 450. In
particular, these changeable images can be used as a dynamic method to
provide access to a password key or as a type of CAPTCHA, (i.e., a
"Completely Automated Public Turing test to tell Computers and Humans
Apart"), where the observer reveals and discovers the hidden pattern 450
or image from one or more of the illusion states. For example, some
letters of a hidden text message can be revealed in one illusion state,
and other letters, in other illusion states. In response, the observer
can provide the CAPTCHA word(s) or image(s), for example as a
verification code to gain limited access to a limited access website.

[0091] Alternately, the observer can provide an input about the visual
qualities or perceptual impact of the optical illusion(s) present in one
or more states of a dynamic optical illusion image, or a series of such
images, as the illusion states of the image(s) change. Exemplary
questions can include: "In what portion of an image is an optical
illusion image present?" or "In what direction does the optical illusion
image appear to move or rotate?" or "Does the image appear to be static,
pulsing, moving, rotating, or scintillating?"

[0092] The two approaches, using hidden patterns 450 embedded in a dynamic
optical illusion image, and asking questions regarding illusion
perception, can also be used in combination for a more difficult reverse
Turing test. As differences in visual perception exist among humans
(e.g., color blindness), the choice of illusions used can be tailored to
different circumstances. By comparison, U.S. Pat. No. 7,929,805 by Wang
et al., entitled "Image-based CAPTCHA generation system," provides for an
image-based CAPTCHA generation system, but it is reliant on distorting
images to hide the human decipherable message, rather than using optical
illusions to do so.

[0093] Dynamic optical illusion images 410 formed in accordance with the
present invention can also be used for a variety of other purpose. For
example, they can be used for applications such as security, stenography,
watermarking, cognitive and visual testing, cultural or consumer
research, interactive clothing or textiles and environmental sensing.
They can also be used for entertainment applications, as in puzzles,
games, artwork and novelty books.

[0094] As discussed previously, some optical illusion images 400,
including some of the physiological variety, require color image content
in order for the optical illusion to be visible. In other cases, even if
the optical illusion may be visible for grayscale versions of the optical
illusion image, the visual effect of the optical illusion is strongly
enhanced by the use of appropriate colors. Other optical illusions do not
rely on the use of color, and will have a substantial visual impact, even
when they are printed in black and white. For example, FIG. 6A depicts an
optical illusion image 400 having a pattern known as the Ouchi illusion
which can be dynamic optical illusion image 410I printed using mutable
inks. The Ouchi illusion is typically printed as a black and white image,
although it also works as a color illusion. The Ouchi illusion has two
patterns of tiled black and white rectangles. The rectangles are arrayed
vertically in a circular central image region 446, and are arrayed
horizontally in the surround image region 447. Other versions of this
illusion, with patterns of tiled lines or with a set of parallel lines
(like a grating) are also available. Typically observers perceive the
inner central image region 446 to float around as the page or the
observer's eyes move. In particular, the central image region 446 is seen
as either floating in front of the surround image region 447, or as a
pattern being viewed through a hole in the surround image region 447.
This sense of depth is considered to arise as a result of the illusory
relative motion. This perception of anomalous motion from this illusion
has not been fully explained, but it involves an interaction between the
edges of the constituent rectangles, their spatial frequency, and eye
motion (saccades). The visual response peaks when the pattern in the
central disk region has a spatial frequency dependence that repeats at
4-7 cycles/degree, with the effect varying with the size of the central
image region 446.

[0095] As a further example, the optical illusion image 400 of FIG. 6A is
adapted to be a dynamic optical illusion image 410I having a first
illusion state 445 and a second illusion state. In that context, FIG. 6A
depicts the first illusion state 445 and FIG. 6B depicts a second
illusion state 448 for the dynamic optical illusion image 410I. In the
depicted second illusion state 448, the constituent rectangles in the
central image region 446 have been altered to be lighter or have less
contrast, as can be accomplished using mutable inks in accordance with
the present invention. For example, the central image region 446 can be
printed with a photochromic ink that has a similar color and contrast to
a normal ink printed in the outer image region 447 when it is in its
activated state. When the photochromic ink is activated, the perception
of the illusion can be strong for an observer. Whereas, when the
photochromic ink is in an unactivated state, the central image region 446
can provide a low contrast pattern (as shown in FIG. 6B) or no pattern at
all. In this case, many observers will perceive a greatly reduced motion
effect. It is noted that an Ouchi pattern can be changed as a dynamic
optical illusion image 410I in other ways to impact the perception of an
illusion, including by changing the central disk region size or the
spatial frequency of the tile pattern.

[0096] The second illusion state 448 of the dynamic illusion image 410I of
FIG. 6B also includes the possibility of hidden image content (e.g.,
hidden pattern 455) becoming visible at the same time that other image
content becomes less visible with application of the external stimulus.
For example, the central image region 446 can be printed with a mixture
of photochromic and thermochromic inks. A mixed stimulus of optical
radiation and thermal stimuli can be applied to independently control the
state of the different image content. For example, the colorant state of
the photochromic ink can be controlled to become more colored while the
colorant state of the thermochromic ink can be controlled to become less
colored, thereby changing the appearance state of the mutable central
image region 446 in order to switch between the first illusion state 445
of FIG. 6A and the second illusion state 448 of FIG. 6B. In this way, the
relative visibility of the hidden pattern 455 or the optical illusion
image content can be independently controlled.

[0097] Where FIG. 1E illustrated the use of a hidden pattern 450
comprising human readable text message, in FIG. 6B, the hidden pattern
455 is a machine readable barcode. The inclusion of hidden patterns in a
dynamic optical illusion image can be generalized such that any sort of
hidden image content can be made to become more visible (to a human
observer or a machine) upon application of an appropriate stimulus by
using mutable colorants. This can occur while other image content,
including illusionary image content, either remains static or also
changes in appearance. Essentially, a sequence of layered images is
accessible within or behind the dynamic optical illusion image. Of
course, an external stimulus such as heat or light can also hide or
reveal hidden image or information content that is outside of or adjacent
to the dynamic optical illusion image 410, as illustrated by adjacent
image content 407 of FIGS. 3A and 5B. Generally, hidden patterns 455 with
a regular or semi-regular patterning, like bar codes or QR codes, can be
hidden better within optical illusion images having regular geometric
patterning, like the Ouchi or the well-known cafe wall style illusions.
These machine readable hidden patterns 455 can not only comprise black
and white patterns, as is shown in FIG. 6B, but variable color patterns
as well. It should be understood that hidden patterns 455 can also be
printed with mutable inks to follow the contours or patterning of the
illusion image, much as shown with the "W" and "A" text characters of
FIG. 1E.

[0098] From the prior discussions, it is clear that optical illusion
images cause perceptual impact by revealing or exploring different
attributes of the human visual system. The presence or absence of color
impacts the effectiveness of some illusions. The spatial frequencies of
the image details, such as the tile pattern in the Ouchi illusion of FIG.
6A, impact the effectiveness of other illusions. Some illusions are more
effective if the image feature edges are straight, abrupt, and regularly
patterned (e.g., the Ouchi illusion of FIG. 6A), while in other illusions
(e.g., the peripheral drift illusion of FIG. 1A), the use of curved edges
is more effective. Likewise, many illusions are dependent on the size of
their field of view. In particular, optical illusion images such as the
peripheral drift illusion of FIG. 1A are preferably large enough,
relative to the field of view of the observer, that part of the image is
perceived by the peripheral vision. Essentially, that means the outer
portions of the image should exceed ˜10° to the eye of the
observer. That occurrence is clearly dependent on the size of the optical
illusion image and the distance between the printed object and the
observer. By comparison, the Ouchi illusion image shown in FIG. 6A, is
optimized for specific spatial frequencies and induces an illusionary
visual response linked to small eye movements (saccades). As a result, it
has the greatest perceptual impact with image sizes that are smaller than
the FIG. 1A optical illusion image, which instead interacts with the
observer's peripheral vision. Thus the printed optical illusion images
should be presented within a size range that will support the perceptual
effects for a target nominal viewing range. Alternately, dynamic optical
illusion images 410 can be modified to provide size or field of view
changes, using mutable inks for example, such that multiple viewing
distances can be supported, or that size changes alter the perceptual
impact.

[0099] In some embodiments, the dynamic optical illusion images 410 are
formed using irreversible mutable colorants that can only undergo a
single state change (e.g., from clear to colored). In this case, the
mutable colorants therein can be subjected to an external stimulus (such
as heat or light) just once to induce changes in image content. In other
embodiments, the dynamic optical illusion images are formed using
reversible mutable colorants that can be repeatedly cycled back and forth
between the different colorant states multiple times in succession.

[0100] In some embodiments the amount of the stimulus that is applied to
the mutable inks can be used to control the amount of color change that
is produced. In this way it is possible to achieve a range of colorant
states providing intermediate colors 360, as suggested in FIGS. 4A-4E, if
so desired. In the above examples, corresponding to FIGS. 1A-1F, FIGS.
2A-2C, and FIGS. 6A-6C, mutable portions 430 of dynamic optical illusion
images 410 and 410A-410I were subject to changes using mutable colorants.
It should be understood that the entirety of a dynamic optical illusion
image 410, as well as adjacent image content 407, can be subject to
changes using mutable colorants.

[0101] For many mutable inks, the color change response time is a few
seconds or less, with some photochromic inks having response times as
small as a microsecond. Therefore, an applied external stimulus to the
printed content can potentially be modulated at video rates (˜30
frames/sec) or faster. However, if different optical illusions associated
with the individual images or frames are to be perceived by an observer,
the modulation rate needs to be reduced, likely to several seconds per
frame or more. In general, the perception of optical illusions requires
some concentration, during which an observer perceives the image content
itself and then the physiological illusionary effect or cognitive
illusionary puzzlement. This process typically takes at least several
seconds or longer. Therefore the temporal cycling between image states
should generally include a prolonged image retention time (of many
seconds per image, if not minutes or longer) before changing the dynamic
optical illusion image. As many color mutable inks are metastable, and
can retain their color changes for minutes or hours once stimulated, this
result is easily achievable. Therefore, the dosage specification for an
external stimulus can include temporal modulation parameters, including
exposure switching times, image retention times, and duty cycles in the
case or cyclic exposures.

[0102] It should be understood that the mutable inks used to exercise the
inventive method for creating and using dynamic optical illusion images
can be advantaged if they have other properties in addition to those
previously discussed. As an example, for some applications, it can be
desirable to allow printed content including dynamic optical illusion
images to change appearance for awhile, but then to have this function
permanently disabled, rendering the ink permanently colored or colorless.
For example, UV radiation can be applied to many chemicals or mixtures,
including some photochromic or thermochromic inks, to break chemical
bonds so that color changes are no longer possible. In the case of
photochromic inks, this disabling light dosage (wavelength, intensity,
and time) needs to be sufficiently distinct from the activating light
dosages that accidental damage is unlikely to occur. As another example,
the increased availability of photochromic or thermochromic inks with
reduced viscosities can better enable inkjet printing of these inks.
Additionally, for some applications, having photochromic inks that
transition from colored to colorless states when the light stimulus is
applied can be useful. Similarly, irreversible thermochromic inks which
provide multiple temperature dependent color states can be useful.

[0103]FIG. 7 shows a flowchart summarizing a dynamic optical illusion
method 700 for providing functional printing of a printed optical
illusion image 725 in accordance with the present invention. A design
dynamic optical illusion image step 707 is used to design a dynamic
optical illusion image 710 having one or more mutable portions. The
mutable portions are designed such that when they are changed from a
first appearance state to a second appearance state, the dynamic optical
illusion image 710 progresses from a first illusion state to a second
illusion state respectively, wherein changing the dynamic optical
illusion image 710 between the first and second illusion states affects
the perception of an optical illusion by a human observer.

[0104] In some embodiments, the dynamic optical illusion image 710 can be
designed starting from an original static optical illusion image 702. One
or more image regions of the static optical illusion image 702 are then
designated to be the mutable portions. First and second appearance states
for the mutable portions are then defined to provide different
appearances for the optical illusion, corresponding to the first and
second illusion states for the dynamic optical illusion image 710.

[0105] As has been described earlier, in some embodiments a hidden pattern
705 can be provided for inclusion in the dynamic optical illusion image
710. The hidden pattern 705 can be a human readable pattern, including
text, or a machine readable pattern that is visible in at least one of
the illusion or appearance states. Changes between appearance states can
also cause changes in the appearance of hidden text or codes within or
adjacent to the optical illusion portion of the dynamic optical illusion
image 710, with or without causing changes in the perceptual impact of
the optical illusion. During the design dynamic optical illusion step
707, the expected appearances of the dynamic optical illusion image 710,
including any hidden patterns 705, for two or more illusion states, can
be previewed using a softcopy display 120 (FIG. 3A).

[0106] The design dynamic optical illusion image step 707 produces a
specification for the dynamic optical illusion image 710, which is then
used in a print optical illusion image step 720, to print the dynamic
optical illusion image 707 on a printer and thus provide a printed
optical illusion image 725. The printed optical illusion image 725 is
printed according to a determined print specification using a plurality
of colorants, including at least one appearance mutable colorants having
spectral characteristics can be controllably switched between a first
colorant state and a second colorant state by application of an
appropriate external stimulus. The specified appearance states of the
mutable portions of the printed dynamic optical illusion image 725 are
achieved by appropriate selection of the colorant levels for the
plurality of mutable colorants, as well as any immutable colorants. The
print specification specifies the colorant levels as a function of
position for the mutable and immutable colorants, which can be optimized
according to the spectral characteristics and other properties of the
mutable and immutable colorants and the printing devices used to produce
the printed dynamic optical illusion image 725. The colorant levels can
be determined via look-up tables, algorithms or other means to provide an
appropriate print specification that will provide the desired illusion
states for the printed dynamic optical illusion image 725. Alternately,
the printing device can receive the specification for the dynamic optical
illusion image 710 and can apply corrections, via look up tables,
algorithms, or other means, to determine the colorant levels. In either
case, the goal is to achieve appropriate image quality attributes,
including color matches, color gamut, color accuracy, the presence of
intermediate colors, metamerism, resolution, or contrast for the printed
optical illusion image 725 in activated and inactivated states.

[0107] An apply external stimulus step 730 is used to apply an appropriate
external stimulus to the printed optical illusion image 725 to produce a
modified optical illusion image 735 having a different illusion state. In
some embodiments, the apply external stimulus step 730 can be replied
repeatedly to modify the illusion state of the printed optical illusion
image 725 a plurality of times.

[0108] In some embodiments, and optional disable optical illusion image
step 740 can be applied, for example using an appropriate UV radiation
dosage or a heat or chemical treatment, to prevent further modulation of
the printed dynamic optical illusion image 725 between illusion states or
appearance states.

[0109] The present invention, relative to creating and using dynamic
optical illusion images 410, has been explained using a limited set of
examples, including a peripheral drift optical illusion (FIGS. 1A-1F), a
cognitive optical illusion that manipulates perspective cues to create
the visual impression of a physically impossible structure (FIGS. 2A-2C),
and a Ouchi optical illusion (FIGS. 6A-6B). However, the functional
printing of dynamic optical illusion images 410 can be applied to a wide
variety of different types of optical illusion images 400. For example,
one or more optical illusion images, including combinations of static and
dynamic optical illusion images, can be printed together, adjacently or
overlapping, within the layout of a larger optical illusion image. While
FIGS. 1A-1F depicted potential changes to a given illusion image, a
dynamic optical illusion image can also be designed to transition from
one optical illusion to another, for example, from a peripheral drift
illusion to scintillating illusion. Also, optical illusion images can be
chosen for perceptual impact, cultural or age sensitivity, color content,
ability to hide text, messages or codes, or for other reasons.

[0110] As the prior discussion has suggested, optical illusion categories
are very broad, and there are many demonstrated optical illusions within
any particular category. As an example, in the paper "Perceiving the
present and a systematization of illusions" by Changizi et al. (Cognitive
Science, Vol. 32, pp. 459-503, 2008), perceptual motion and size
illusions were parsed into 28 classes or groupings. It should be
understood that the inventive method for creating and using dynamic
optical illusion images 410 includes modifying optical illusion images
400 selected from a group including, but not limited to, geometrical or
spiral illusions, anomalous motion illusions, rotational illusions, color
change illusions, peripheral drift illusions, positive after image
blurring illusions, scintillation and grid illusions, convection,
contraction and expansion illusions, contrast polarity illusions,
perspective illusions, or variations and combinations thereof. While
these illusions cannot all be depicted here, the method of the present
invention can be applied to many optical illusions that are known in the
art, which include, but are not limited to the Hering illusion, the
Orbison illusion, the Delboeuf illusion, the Muller-Lyer illusion, the
Wundt Illusion, the Ehrenstein illusion, the Hermann grid Illusion, the
scintillating grid illusion, the pulsing vortex illusion, the
Fraser-Wilcox illusion, the Ouchi illusion, the peripheral drift
illusion, the flowing leaves illusion, the waves illusion, the moving
bulge illusion, the Cafe Wall illusion, the rotating snakes illusion, the
Kanizsa Triangle illusion, the Munker-White Illusion, Adelson's checker
shadow illusion, comparative color illusions, the Zollner Illusion, the
Ponzo illusion, or impossible figures such as the Penrose Triangle or the
Penrose Stairs, or variations and combinations thereof.

[0111] It is noted that U.S. Pat. No. 7,136,522 by S. Harrington et al.
entitled "Systems for spectral multiplexing of source images to provide a
composite image, for rendering the composite image, and for spectral
demultiplexing of the composite image to animate recovered source
images," provides an alternative method of intelligent printing that
provides a composite image having a multitude of source images printed
within it using a plurality of colorants or inks With this approach, the
individual source images can be rendered distinguishable when the
composite image is subjected to illumination by narrow band illuminants.
However, the inks are normal process inks rather than chromogenic or
mutable inks. The printed composite image can appear different to an
observer based on the spectral absorption and reflection of the inks with
respect to the illuminating light spectrum, in order to reveal different
hidden source images. The rendered composite image may be subjected to a
varying illumination, including by a switched sequence of individually
activated, differing illuminants to cause a corresponding sequence of
source images to be recovered in rapid succession. Harrington does not
provide that the either the composite image or source images are optical
illusion images.

[0112] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within the
scope of the invention. It is emphasized that the apparatus or methods
described herein can be embodied in a number of different types of
systems, using a wide variety of types of supporting hardware and
software. It should also be noted that drawings are not drawn to scale,
but are illustrative of key components and principles used in these
embodiments.

[0113] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within the
spirit and scope of the invention.